Chromium plating is used for decoration and for industrial use in many industrial fields. Chromium plating is widely used as decoration plating because it does not corrode in the air and does not lose luster. Also, chromium plating is widely used for mechanical parts and the like that require wear resistance because it has high hardness and a low friction coefficient. A large amount of hexavalent chromium is used in a plating liquid used for this plating. Since the effect of hexavalent chromium on the human body is feared, hexavalent chromium must be reduced to trivalent chromium under very strict conditions so that the hexavalent chromium is not released in the environment in the treatment of the plating waste liquid. Therefore, the development of a plating liquid using trivalent chromium, instead of hexavalent chromium, is desired.
For the plating liquid using trivalent chromium, for example, Patent Document 1 describes a chromium plating liquid using a trivalent chromium compound, such as chromium chloride, chromium sulfate, and chromium sulfamate, as a plating liquid for decoration plating. However, when an inorganic salt of trivalent chromium, such as chromium chloride or chromium sulfate, is used as a source of chromium, chromium is consumed by plating, while chloride ions or sulfate ions, which are the counteranions of the chromium salt, remain in the plating liquid. From the necessity to keep the liquid composition of the plating liquid constant, the plating liquid is used by appropriately adding the source of chromium in an amount corresponding to the consumed chromium, and therefore, chloride ions or sulfate ions are accumulated in the plating liquid. Therefore, finally, the liquid composition cannot be kept constant, and thus, the total amount of the plating liquid is replaced by a new plating liquid, and the spent plating liquid is treated as a waste liquid.
As a process for solving this problem, Patent Document 2 proposes a trivalent chromium plating process in which when trivalent chromium plating is performed using a plating liquid containing chromium chloride and ammonium chloride, part of the plating liquid is circulated in a cooling apparatus, and part of the ammonium chloride is crystallized and removed in this cooling apparatus to perform plating while controlling the concentration of the ammonium chloride in the plating liquid.
Also, solving this problem by using, as a source of trivalent chromium, chromium hydroxide, which is a compound without counteranion accumulation, in the state of a water-containing gel is proposed (see Patent Document 3). However, chromium hydroxide generally has low solubility in water and also has low solubility in an acidic aqueous solution used as an ordinary plating liquid. Therefore, long-time stirring under heating is required for the preparation of a plating liquid. Also, when the consumed chromium is supplemented, a long time is required to dissolve supplied chromium hydroxide. For these reasons, the plating work is interrupted during that time, and problems occur in the preparation of the plating liquid and the plating work.
For example, processes described in Patent Documents 4 to 6 are known as conventional processes for producing chromium hydroxide. But, these documents do not mention using chromium (III) carbonate as a source of trivalent chromium, instead of chromium hydroxide.
Also, the following Non-Patent Document 1 describes that chromium (III) carbonate is obtained by adding an alkali carbonate or alkali hydrogencarbonate solution to an aqueous solution of a chromium (III) salt. The document describes that this chromium (III) carbonate is light green. Chromium (III) carbonate obtained in this manner is a precipitate as described in the document, and therefore, it cannot be used as a source of trivalent chromium.